Self-switched inductive fault current limiter

ABSTRACT

A parallel-connected inductance and capacitor are connected in parallel with a normally closed switching device and in series with an electric power distribution system. The capacitive reactance of the capacitor is substantially higher than the inductive reactance of the inductor at the power line frequency. The normally closed switching device is opened in response to a predetermined level of fault current in the system so that the reactor is inserted into the power distribution system to effect limitation of current level. The switching device is springbiased to a closed position and the current path through the switch contacts forms a blow-open magnetic circuit, whereby the contacts are blown open against the closing force of the biasing springs when the predetermined current magnitude is reached. Means are provided to produce a high arcing voltage in a pressurized liquid medium.

1 Dec. 16, 1975 SELF-SWITCHED INDUCTIVE FAULT CURRENT LIMITER [75]Inventor: Lorne D. McConnell, Chalfont, Pa.

[73] Assignee; I-T-E Imperial Corporation, Spring House, Pa.

[22] Filed: Apr. 22, 1974 [21] Appl. No.: 462,781

[52] US. Cl. 317/11 C; 317/11 E; 317/11 A; 317/11 R; 317/16; 200/144 AP[51] Int. Cl. H0211 7/22 [58] Field of Search 317/16, 18 R, 20, 53, 11C,317/11 E, 11 A, 11 R, 11 B, 11 D; 200/144 AP, 146 R; 307/135, 136;323/76, 43.5

[56] References Cited UNITED STATES PATENTS 1,755,111 4/1930 Gay 317/531,894,119 1/1933 Pratt 317/20 2,350,195 5/1944 Rypinski 317/20 2,374,9745/1945 Blume 307/135 2,977,488 3/1961 Walz et al.... 307/136 X 3,192,3386/1965 Haynes 317/20 X 3,192,440 6/1965 Baltensperger 200/144 AP X3,219,883 11/1965 Barkan 317/20 X 3,289,042 11/1966 Bodenschatz....317/20 X 3,376,475 4/1968 Greber 317/11 B 12/1973 Kroon 335/16 9/1974Clausing 200/144 AP X Primary ExaminerJ. D. Miller AssistantExaminerlatrick R. Salce Attorney, Agent, or FirmOstrolenk, Faber, Gerb& Soffen [s7] ABSTRACT A parallel-connected inductance and capacitor areconnected in parallel with a normally closed switching device and inseries with an electric power distribution system. The capacitivereactance of the capacitor is substantially higher than the inductivereactance of the inductor at the power line frequency. The normallyclosed switching device is opened in response to a predetermined levelof fault current in the system so that the reactor is inserted into thepower distribution system to effect limitation of current level. Theswitching device is spring-biased to a closed position and the currentpath through the switch contacts forms a blow-open magnetic circuit,whereby the contacts are blown open against the closing force of thebiasing springs when the predetermined current magnitude is reached.Means are provided to produce a high arcing voltage in a pressurizedliquid medium.

7 Claims, 11 Drawing Figures US. Patent Dec. 16, 1975 Sheet2of53,927,350

FIE-.4-

US. Patent Dec.16,1975 Sheet40f5 3,927,350

SELF-SWITCHED INDUCTIVE FAULT CURRENT LIMITER RELATED APPLICATIONSBACKGROUND OF THE INVENTION This invention relates to current limitingdevices, and more particularly relates to a fault current limiter forelectric power systems. By electric power systems is meant powertransmission and power distribution systems operating at from 50 to 60hertz, and having voltage ratings in excess of about 10,000 volts andcurrent ratings in excess of about 500 amperes. However, the concepts ofthe invention are also applicable to lower voltage systems.

As the power of electric power transmission and distribution systemsincreases, the problems of system stability during fault and the extentof possible fault damage also increases. The consequent increase inrequired circuit breaker performance and in performance of otherequipment in handling high fault levels in such systems hassubstantially increased the cost of the breakers and associatedequipment. Thus, devices which are capable of inserting a currentlimiting impedance into the power circuit, with the initiation of afault condition in the circuit, are increasingly desirable since theyallow the use of equipment such as breakers and the like which need notbe capable of withstanding the available short circuit capacity of thesystem (shortcircuit currents which could be reached in the absence ofthe current limiting impedance).

Fault current limiting devices have been employed in the past whichconsist of fan inductance-capacitance series circuit which is resonantat the power line frequency. Under afault condition, and at a selectedinstantaneous current (and a selected instantaneous voltage across thecapacitor) a saturable reactor or high impedance in parallel with thecapacitor saturates or switches to a relatively low impedance to shuntthe capacitance. The inductor is then left in the circuit to provide anadditional current limiting inductance in series with the source. I

In the above prior art system, the series capacitor is physically largeand is very expensive. Moreover, series capacitors in electric powersystems are not reliable and are not desired by power utilities.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the presentinvention, a fault current limiter is formed of a parallel-connectedcapacitor andinductor which are in series with the power line, but arenormally. short circuited by a normally closed parallel-connectedswitching device. The switching device is operated to an open positionwhen the power line fault current reaches a given value. Preferably, theswitching device is self-operated at the given fault current, as byemploying a blow-open configuration for the switch contact current pathwith the contacts held closed againt the blow-open force by a givenspring closing force.

Other switch arrangements can be used. in accordance with the broadconcept of the present invention.

In each case, fault current sensing switch operation and cut-in of thehigh impedance element must take place in a time period. of the order of2 milliseconds in order that the first crest of a-c current may belimited in magnitude. This action could be obtained by using a currenttranformer to monitor current level; solid state circuitry to processthe output of the current transformers and to deliver an output forinitiating the opening of the switch means; an electrodynamic type drivecircuit to deliver interrupter opening energy; and a mechanical switchoperable by the electrodynamic drive to open the normal current path anddevelop a high enough arc voltage drop to effect commutation of thecurrent into the current limiting impedance.

The parallel capacitor of the novel circuit serves two purposes. First,it facilitates commutation of the current from the normal current paththrough the switch contacts and into the inductor when the switchcontacts open. Secondly, the capacitor serves as a transient voltagedamping means across the inductor upon the interruption of the faultcurrent by the system circuit breaker. It is also necessary that thecapacitor is sized to avoid parallel resonance with the inductor atpower system frequency. Thus, the capacitive reactance of the capacitorshould be equal to or greater than about 10 times the inductivereactance of the inductor at power system frequency.

The novel fault current limiter of the invention has the advantages oflow cost and reliability, as compared to prior art systems. The devicehas the same size as would be required by the series reactor alonesince, as will be seen, the various components can be mounted within thevolume defined by the inductor coil. The device is also simpleand'requires no auxiliary power for its operation. The unit is thusself-contained and may be self-operating and all equipment can bemounted at line potential.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of aparallel-connected capacitor and inductor and switch constructed inaccordance with the invention.

FIG. 2 is an end view of the fault current interrupter constructed inaccordance with the invention.

FIG. 3 is a cross-sectional view of FIG. 2 taken across section line 33in FIG. 2.

FIG. 4 is an end view of a first embodiment of an interrupter switch ofthe invention in which the contacts are butt contacts formed onconfronting pistons.

FIG. 5 is a cross-sectional view of FIG. 4 taken across the section line55 in FIG. 4.

FIG. 6 is a cross-sectional view of a single break interrupter switchmade in accordance with the invention and is a cross-sectional view ofFIG. 7 taken across the section line 66 in FIG. 7.

FIG. 7 is a cross-sectional view of FIG. 6 taken across FIG. 11 is across-sectional view of FIG. 10 taken across section line 11-11 in FIG.10.

DETAILED DESCRIPTION OF THE DRAWINGS Referring first to FIG. 1, thenovel invention is shown in diagram fashion and contains terminals 21and 22 which are connected in series with a conventional power circuitbreaker and a conventional electric power transmission circuit. Thefault current limiter, in accordance with one aspect of the invention,consists of parallel-connected inductor 23, capacitor 24 and switchdevice 25.

Switch device 25 will be described later in connection with FIGS. 3 to 11 and operates normally to shortcircuit inductor 23 and capacitor 24.Thus, the switching device 25 has a schematically illustrated movablecontact member 26 which engages fixed contact 27 under normal circuitconditions. When a fault current flows in the circuit being protectedand as will be later described, the contact arm 26 moves toward contact28 and an arc current flows from contact 27, througharc-current-limiting impedances 29 and 30 (which are connected to oneanother through the arc) and to movable contact member 26. The arcvoltage which appears across contacts 26 and 27 causes current from themain circuit to commutate into capacitor 24 which initially acts as alow impedance to current change and thence into inductor 23, which willthen exert a current limiting action on the main circuit current. Themain current is ultimately interrupted by the relatively slow maincircuit breaker 20.

In one embodiment of the invention, the inductor 23 is an air coreinductor having an inductive reactance at 60 hertz of the order of 1ohm. The capacitor 24 facilitates the commutation of current into theinductor 23 and also serves as a transient voltage damper in the system.Preferably, the capacitor 24 will have a capacitive reactance of 10 ohmsor more at 60 hertz.

FIGS. 2 and 3 show one embodiment of the fault current limiter ofFIG. 1. The terminals 21 and 22 may have any configuration to allow easyconnection of the component into an existing power system. The device ofFIGS. 2 and 3 is contained within a weatherproof glass filament-woundand epoxy impregnated cylindrical housing which has molded or castinsulation end closures 41 and 42 sealed thereto. The inductor 23 isthen wound on an insulation cylinder or bobbin 43 which is suitablyfixed within housing 40. The end terminals 45 and 46 are then suitablyconnected to terminals 21 and 22. A plurality of parallel-connectedcapacitors, including capacitors 48 and 49, are also connected by theirleads 50-51 and 52-53 respectively to terminals 21 and 22 as shown.Capacitors 48 and 49 in FIG. 3 constitute the capacitor 24 of FIG. 1.

Switching device 25 is then connected as shown between terminals 21 and22 and in parallel with inductor coil 23 and capacitors 48 and 49. Theinterior of tube 40 may then be filled with air, or may be filled with agas such as sulfur hexafluoride, if desired, to improve the insulationof the inductor 23, switch 25 and capacitors 48 and 49. Mounting legs 55and 56 may be provided to allow the easy mounting of the device in thepower system. v

FIGS. 4 and 5 show a first embodiment of the selfoperating switch 25 ofFIGS. 1 and 3. Switch 25 has terminals 60 and 61 which are connected toor might be terminals 21 and 22, respectively, in FIG. 3. The terminals60 and 61 are in turn connected to conductive end caps 62 and 63,respectively, which are threaded onto the opposite ends of insulationtube 64.

Terminal 61 has an elongated thread which enters threaded opening 65 inend cap 63 and has an enlarged head 66 which threadably receivesconductive cylinder 67. A nut 68 secures terminal 61 and its head 66 tothe cap 63 and the entire terminal 61 can be secured in a higher orlower position for spring load adjustment, as will be later described.An O-ring seal 69 seals the outer surface of cylinder 67 to the innersurface of tube 64.

Cylinder 67, which is fixed in position, has a plurality of openings inits top, including openings 70 and 71, and has a butt-contact member 72secured to and extending from its axial center as shown in FIG. 5.Contact member 72 cooperates with butt contact 75 which is carried onconductive movable piston 76. Piston 76 is axially movable along itsaxis and along the axis of tube 64. A compressed spring 77 biases piston76 downwardly to press contact 72 and 75 toward engagement with oneanother with a given force. A conductive ring 78 is electrically andmechanically secured to cap 62, and a conductive and flexible shunt,schematically shown as shunt elements 79 and 80, electrically connectpiston 76 to ring 78 and thus to cap 62 and terminal 60.

The cap 62 has an opening 81 therein and a conductive tube 82 is securedto cap 62. The tube 82 is then filled with brass or copper screening andis capped with a conductive end cap 91 which is threaded onto thethreaded end of tube 82. Cap 91 has a central plug 92 therein whichpermits easy access through the cap 91 to the interior of tube 82. Thetube 82 serves both as a terminal conductor and as a filling spout forfilling the interior of insulation tube 64 with liquid SF, to the levelindicated in FIG. 5. The system is then charged with SF gas at apressure of about 300 p.s.i.g. The liquid sulfur hexafluoride thenoperates in such a manner as to generate SF gas at higher pressure whena high voltage arc is drawn between contacts72 and 75. Note that SFliquid and charging SF gas could be replaced by other liquids and gases,and by combinations of diverse liquids and gases so long as the liquidis one which will produce a high are voltage. The tube 82 and theportion of ring 78 above the liquid level serves as an expansion spacefor expanding gas developed during arcing, while screening 90 serves asa cooling means to rapidly cool and recondense SF gas generated by theliquid during arcing.

The operation of the interrupter of FIGS. 4 and 5 is as follows:

A normal current path is established through the normally closed buttcontacts 72-75 which extends from terminal 61, head member 66, cyliner67, butt contact 72, butt contact 75, movable piston 76, shunt members79 and 80, conductive ring 78, cap 62 and tube portion 82 of terminal60. It will be noted that the current flow through piston 76 and theninto butt contact 72 makes an approximately 90 turn as it comes from theupper surface of piston 76 into butt contact 72. Similarly, currentthrough butt contact 75 makes an approximate right-angle turn whenflowing through the lower surface of piston 76 and then outwardly and upthe piston walls. Theseturns then define a blow-off path in whichmagnetic forces tend to move contacts 72 and 75 away from one another.Thus, when the current becomes sufficiently high, the blow-off forcebecomes sufficient to move piston 76 upwardly toward a stop positionagainst the bottom of ring 78 and against the downward biasing force ofcompression spring 77. The piston 76 further serves as a dashpot todelay the reclosing of contacts 72 and 75 when the blow-open forcedecreases.

The point at which contact separation between contacts 72 and 75 takesplace can be controlled by adjusting the compression of spring 77through the adjustment of conductive member 61 and after loosening thenut 68. Once the contacts separate because of the given instantaneouscurrent flowing in the blowoff path, an arch will be drawn betweencontacts 72 and 75 which can heat the liquid SF sufficiently high toproduce sulfur hexafluoride gas. This gas bubble of expanding hot gaswill provide a pressure level for expelling piston 76 away from piston67, further lengthening the arc, and developing high arc voltage. Notethat the arc voltage will cause current to commutate into the parallelcapacitor and thence into the current limiting inductance 23 so that thenetwork fault current will be' limited in magnitude. The current isultimately interrupted by the conventional circuit breaker whichresponds relatively slowly to the appearance of the fault current in thecircuit.

Once the fault current is extinguished by the circuit breaker 20, thebutt contacts 72 and 75 of FIG. 5 are reclosed under the influence ofspring 77. Both cylinder 67 and piston 76 in FIG. 5 have openings suchas openings 70 and 71 to permit the flow of fluid and of gastherethrough, while the openings in piston 76 control its dashpotcharacter.

The petcock 92 in FIG. 5 provides means for filling the interior of tube64 with SP liquid and the copper or bronze screen 90 serves as aneffective cooling chamber for recondensation of gaseous SF into liquid.

FIGS. 6 and 7 show a second embodiment of the self-switching device ofthe invention. Referring to these figures, the self-switching device(FIG. 1) consists of a glass-epoxy reinforcing tube 100 which has abottom closure 101 and a top closure 102 threadably secured to its upperand lower ends. A molded epoxy housing 103 is then positioned withintube 100 and is sealed at its bottom to the bottom cap 101 by the sealring 104. A catch basin 105, which serves to trap solid arc products, isalso positioned at the bottom of housing 103.

The conductive terminals of the device consist of terminals 106 and 107which correspond to terminals 21 and 22, respectively, in FIG. 3.Terminal 107 has an upwardly extending contact end 108 which is fixed inposition while terminal 106 has a rotatable contact member 109 pivotallyconnected thereto on the fixed pivot 110. Pivot 110 can be currentcarrying but, preferably, flexible conductive shunts (not shown) carrycurrent around pivot 110.

A biasing spring 111 is then secured within housing 103 and pressesagainst contact finger 109 so that the contact finger 109 is normallypressed into engagement with fixed contact member 108. Damping means mayalso be connected to the spring 111 to delay reclosing of the contact109 after it opens. A conductive arc runner 1 12 is physically connectedto terminal 106 and thus to movable contact 109 while a second arcrunner 113 is connected to stationary contact 108 and terminal 107. Eachof arc runners 112 and 113 are further coated with appropriate carbon orout-gased activated carbon arc runners 114 and 115, where these are runners 114 and 115 may be of either linear or nonlinear resistivematerial.

The container or outer tube is then filled with liquid SF to the levelindicated by line 120. The entire switching device may then be orientedsuch that the magnetic field of reactor coil 23 in FIG. 3 will take thedirection of arrow 121 in FIG. 7 to increase the blowoff force on thearc between runners 114 and 115.

The device of FIGS. 6 and 7 operates in the following manner:

The current path through the device makes a U- shaped bend when goingfrom terminal 106 into movable contact 109 and then down through contact108 and out the terminal 107. This U-shaped path then develops ablow-off force which tends to rotate rotatable contact 109 in acounterclockwise direction. This rotation, however, is opposed by thebiasing force of spring 111. However, once the instantaneous currentmagnitude through the device is sufficiently high, the contact 109 willrotate counterclockwise against spring 111, and an arc will be drawnbetween the separating contacts 109 and 108. Upon establishment of thearc, a high pressure gas bubble forms between the faces of contacts 108and 109, providing further opening force to contact 109. This are willbe transferred to the arc runners 114 and in the conventional fashionand, as the arc progresses upwardly along the runners, increasedresistance is inserted in the arcing circuit, thereby to increase thevoltage between terminals 106 and 107. If desired, a nonlinearresistance material can be used for the arc runners 114 and 115, such aspure iron or pure tungsten, or the like, which exhibits a dramaticincrease of resistivity with heating due to the arc current.

It is to be noted that a suitable retarding force (not shown) should beapplied to the contact 109 to prevent it from reclosing once the arccurrent is transferred to the arc runners 114 and 115. To this end, asuitable dashpot or the like can be connected to contact 109 so that itrecloses only after the main circuit has been cleared of the faultcurrent. This can be accomplished by a relatively small damping bellowsor the like since the delay time needed is extremely short, for example30-50 milliseconds. Note further that the dielectric medium liquid levelis above the level of the arc runners 114 and 115.

If desired, arc splitter plates may extend downwardly from closure 102and between the arc runners 1 14 and 115 to ensure further elongation ofthe are as it traverses upwardly between the runners 114 and 115.

FIGS. 8 and 9 show a still further embodiment of the self-switchingfault current limiter switch of the invention which uses a double breakconfiguration. The device of FIGS. 8 and 9 consists of a tubular housingwhich contains a stack of insulation plates shown as plates 131 to 138which are spaced from one another and from end plugs 139 and 140 byspacer rings 141 to 148. Each of the plates 131 to 138 are contoured todefine completely cutaway side sections which are aligned with oneanother along with increased radial thickness sections which are alsoaligned with one another above the center line of tube 130 in FIG. 9.The central plates 134 and 135 may be joined at a portion of theiropposing surfaces by a bridging section 150, whereby the various platesdefine staggered and tortuous flow channels through the plates for gaswhich is produced upon the operation of the contacts as will be laterdescribed.

The device terminals and 161 enter through end plugs 139 and 140,respectively, and terminate with upraised fixed contact portions 162 and163, respectively. The bridging contact 164, which is carried onoperating rod 165, is then movable into and out of engagement withstationary contact ends 162 and 163.

The operating rod 165 extends downwardly into a damping assembly 166carried within cup 167. A biasing spring 168 then presses the dampingassembly piston 166 downwardly, thereby to bias movable bridging contact164 into engagement with contacts 162 and 163. The contacts 162 and 163are then provided with conductive arc runners 170 and 171, respectively,which have carbon arc runner material on their upper surfaces to definecarbon arc runners 172 and 173, respectively. The entire tube 130 isthen filled with liquid SF to the level indicated in the upper cup 179in FIG. 9 which communicates with the interior of tube 130 throughopening 130a.

In operation, the current through the device 25 of FIG. 9 travels aU-shaped path in going from stationary contact 162 through the bridgingcontact 164 and into the stationary contact 163. Thus, a blow-off pathis defined, such that when the current magnitude in the path issufficiently high, say two to three orders of magnitude greater thanrated current, the blow-off force exceeds the biasing force of spring168 so that contact 164 moves upwardly and an arc is drawn to thestationary contacts 162 and 163.

Note that the contact 164 will tend to remain open for a time dependingupon the damping action provided by damping piston 166 after the openingforce diminishes or disappears. Note further that the contact 164 can beflexibly connected to the operating rod 165 .to ensure good contactengagement with the stationary contacts 162 and 163.

The arc plates or are splitters 131 to 138 and the bridging section 150are selectively provided with openings therethrough to permit the flowof gas produced from sulfur hexafluoride liquid into the expansionregion above the SP liquid level in cup 179. Thus, during operation,when the current between terminals 160 and 161 becomes sufficientlyhigh, contact 164 moves upwardly and arcs are drawn from contact 164 tostationary contacts 162 and 163. These arcs then transfer to the arcrunners 172 and 173, respectively, and run outwardly along arc runners172 and 173, thereby to be elongated and cooled by the arc splitters.Note that the arc splitters 131 to 138 can be formed of a suitableceramic or carbon composition of any desired type. The arc voltagesproduced are then relatively high and cause commutation of current intothe inductor coil 23 in FIGS. 1 and 3. Thus, the main current fault islimited in magnitude until the circuit breaker 20 is operated. Note thatthe damping applied to the contact 164 is such that the contact does notreclose on the contacts 162 and 163 until after the main current iscleared by circuit breaker 20 (FIG. 1).

FIGS. and 11 show a still further embodiment of the invention with adouble break arrangement. Thus, in FIG. 11 a glass epoxy tube 180,having a top closure 181 and a molded epoxy bottom insert cup 182,receives two extending terminals 183 and 184 which correspond toterminals 21 and 22, respectively, in FIG. 3. Each of terminals 183 and184 terminate in stationary contacts 185 and 186, respectively, whichare bridged by a movable bridging contact 187. The movable bridgingcontact 187 is then carried in a slot 188 in insulation rod 189 and aleaf spring 190 presses contact 187 downwardly in slot 188. The rod 189then 8 has a piston-shaped end 191 which is received in a cup 192 whichis secured to the bottom of insulation cup 182.

A compression spring 192a then biases the rod 189 downwardly, thereby tocause the bridging contact 187 to engage stationary contacts and 186with a given contact pressure. Note that piston 191 serves the func tionof a damping assembly to delay the reclosing of contact 187 after thecontact is opened.

Each of terminals 183 and 184 are then provided with respective arcrunners 193 and 194 which extend upwardly and which extend on eitherside of a central arc runner 195. The central arc runner 195 issupported by a threadably adjustable support rod 196 which is carried inthe closure 181 so that the height of the central arc runner 195 can beadjusted without opening closure 181.

In operation, the current flow through terminals 183 and 184 takes aU-shaped bend through bridging contact 187. Thus, a blow-off force isapplied to the contact 187 which is opposed by the biasing force ofspring 192a. Once this biasing force is exceeded, however, contact 187moves upwardly and the entire rod 189 will move upwardly with thecontact 187 and arcs are drawn from contacts 185 and 186 to the movablecontact 187. These arcs will then be transferred in the usual manner toarc runners 193 and 194 and the arc ultimately, as it expands upwardly,will move off contact 187 and seat on either side of arc runner 195.

Two arcs will then continue to move upwardly, one extending from runner193 to runner 195 and the other extending from runner 195 to runner 194.A relatively high voltage drop then appears across terminals 183 and184, thereby to cause current flow into the current limitng inductance23 of FIGS. 1 and 3.

Although this invention has been described with respect to preferredembodiments, it should be understood that many variations andmodifications will now be obvious to those skilled in the art, and it ispreferred, therefore, that the scope of the invention be limited, not bythe specific disclosure herein, but only by the appended claims.

The embodiments of the invention in which an exclusive privilege orproperty is claimed are defined as follows:

1. A fault current limiter for connection in an electric power circuit,and in series with a circuit breaker in said electric power circuit;said fault current limiter comprising an inductor, a capacitor, and aswitching device; said switching device having interrupting capability;said inductor, capacitor and switching device being connected inparallel with one another; said switching device being operable betweenan open circuit condition and a closed circuit condition; operatingmeans associated with said switching device and coupled to the currentin said electric power circuit for maintaining said switching device inits said closed circuit condition when a circuit parameter in saidelectric power circuit is at or below a given value, and for operatingsaid switching device to its said open circuit condition when saidcircuit parameter exceeds said given value; said electric power circuithaving a given frequency; said capacitor and said inductor beingresonant at a frequency substantially higher than said given frequency;said electric power circuit frequency being from 50 to 60 hertz; saidcapacitor having a capacitive reactance of about 10 times or more thanthe inductive reactance of said inductor.

2. The fault current limiter of claim 1 wherein said circuit parameteris current, and wherein said given value for said circuit parameter is acurrent magnitude which is above the rated current of said electricpower circuit.

3. The fault current limiter of claim 2 wherein said switching deviceincludes a pair of cooperable contacts movable between an open positionand a closed position to define said open and closed circuit conditionsrespectively, and biasing means for biasing said pair of cooperablecontacts to an engaged position.

4. The fault current limiter of claim 3 wherein said inductor comprisesan air core winding; a hollow cylindrical housing; said air core windingbeing supported within the body of said hollow cylindrical housing; saidcapacitor and said switching device being contained within said housing;said housing being sealed and filled with a dielectric gas. 7

5. The fault current limiter of claim 3 wherein the current path throughsaid pair of cooperable contacts defines a blow-off path whereby saidpair of cooperable contacts are moved to a disengaged position againstthe force of said biasing means when the current through said contactsexceeds said given value; and delay means connected to said contacts fordelaying the reclosing of said contacts for a given time following theiropening.

6. The fault current limiter of claim 1 wherein said switching deviceincludes a pair of cooperable contacts movable between an open positionand a closed posi- 10 tion to define said open and closed circuitconditions respectively; and biasing means for biasing said pair ofcooperable contacts to an engaged position.

7. A fault current limiter for connection in an electric power circuit.and in series with a circuit breaker in said electric power circuit;said fault current limiter comprising an inductor, a capacitor, and aswitching device; said inductor, capacitor and switching device beingconnected in parallel with one another; said switching device beingoperable between an open circuit condition and a closed circuitcondition; operation means associated with said switching device andcoupled to the current in said electric power circuit for maintainingsaid switching device in its said closed circuit condition when acircuit parameter in said electric power circuit is at or below a givenvalue, and for operating said switching device to its said open circuitcondition when said circuit parameter exceeds said given value; saidelectric power circuit having a given frequency; said capacitor and saidinductor being resonant at a frequency substantially higher than saidgiven frequency; said inductor comprising an air core winding; a hollowcylindrical housing; said air core winding being supported within thebody of said hollow cylindrical housing; said capacitor and saidswitching device being contained within said housing; said housing beingsealed and filled with a dielectric gas.

1. A fault current limiter for connection in an electric power circuit,and in series with a circuit breaker in said electric power circuit;said fault current limiter comprising an inductor, a capacitor, and aswitching device; said switching device having interrupting capability;said inductor, capacitor and switching device being connected inparallel with one another; said switching device being operable betweenan open circuit condition and a closed circuit condition; operatingmeans associated with said switching device and coupled to the currentin said electric power circuit for maintaining said switching device inits said closed circuit condition when a circuit parameter in saidelectric power circuit is at or below a given value, and for operatingsaid switching device to its said open circuit condition when saidcircuit parameter exceeds said given value; said electric power circuithaving a given frequency; said capacitor and said inductor beingresonant at a frequency substantially higher than said given frequency;said electric power circuit frequency being from 50 to 60 hertz; saidcapacitor having a capacitive reactance of about 10 times or more thanthe inductive reactance of said inductor.
 2. The fault current limiterof claim 1 wherein said circuit parameter is current, and wherein saidgiven value for said circuit parameter is a current magnitude which isabove the rated current of said electric power circuit.
 3. The faultcurrent limiter of claim 2 wherein said switching device includes a pairof cooperable contacts movable between an open position and a closedposition to define said open and closed circuit conditions respectively,and biasing means for biasing said pair of cooperable contacts to anengaged position.
 4. The fault current limiter of claim 3 wherein saidinductor comprises an air core winding; a hollow cylindrical housing;said air core winding being supported within the body of said hollowcylindrical housing; said capacitor and said switching device beingcontained within said housing; said housing being sealed and filled witha dielectric gas.
 5. The fault current limiter of claim 3 wherein thecurrent path through said pair of cooperable contacts defines a blow-offpath whereby said pair of cooperable contacts are moved to a disengagedposition against the force of said biasing means when the currentthrough said contacts exceeds said given value; and delay meansconnected to said contacts for delaying the reclosing of said contactsfor a given time following their opening.
 6. The fault current limiterof claim 1 wherein said switching device includes a pair of cooperablecontacts movable between an open position and a closed position todefine said open and closed circuit conditions respectively; and biasingmeans for biasing said pair of cooperable contacts to an engagedposition.
 7. A fault current limiter for connection in an electric powercircuit, and in series with a circuit breaker in said electric powercircuit; said fault current limiter comprising an inductor, a capacitor,and a switching device; said inductor, capacitor and switching devicebeing connected in parallel with one another; said switching devicebeing operable between an open circuit condition and a closed circuitcondition; operation means associated with said switching device andcoupled to the current in said electric power circuit for maintainingsaid switching device in its said closed circuit condition when acircuit parameter in said electric power circuit is at or below a givenvalue, and for operating said switching device to its said open circuitcondition when said circuit parameter exceeds said given value; saidelectric power circuit having a given frequency; said capacitor and saidinductor being resonant at a frequency substantially higher than saidgiven frequency; said inductor comprising an air corE winding; a hollowcylindrical housing; said air core winding being supported within thebody of said hollow cylindrical housing; said capacitor and saidswitching device being contained within said housing; said housing beingsealed and filled with a dielectric gas.